Patent GB-831682-A (GB)
Title: Improvements relating to sorting
Inventor: TOMBS DAVID MARTINEAU
IPC: B07C5/36
Filed: 1955-03-03
Abstract: 831,682. Photo-electric and like sorting arrangements. NATIONAL RESEARCH DEVELOPMENT CORPORATION. May 11, 1956 [March 3, 1955], No. 6329/55. Class 40 (3). [Also in Groups XXX and XXXVI] In the sorting of bulk material, e.g. nodules of limestone containing a small proportion of nodules containing lead, the material is delivered from a hopper 5 to the trays 2 of an endless conveyer 1. The number of nodules in each tray may be determined by weighing and is set in accordance with statistical calculations. The trays are pivoted to each other at points along extension links 3 and pass round drums 4. Each tray has a bottom formed of parallel spaced rods mounted in a frame hinged at the front end of the tray. The bottoms are normally held in position by spring-loaded latches at the rear of the trays. As the trays move past a transverse row of stationary photosensitive cells 6 the batch is viewed and if it contains an unwanted nodule the signal derived from the cells is used to " mark " the tray for tripping so that its contents are delivered to a second conveyer 7 and not to the hopper 8 with the desired material. The latch mechanism of each tray includes a transversely mounted bar magnet of a material having a substantially square B-H loop. This, as the conveyer moves, passes between the poles of three horseshoe magnets. One of these, 26, is a permanent magnet and gives all the bar magnets a standard polarity. Magnet 24 is a weak magnet, not by itself sufficient to affect the polarity of the bar magnets but carrying a coil 27. The coil 27 is energized in response to the photo-electric detection of unwanted material in a tray at the time when the tray's bar magnet is passing through the polepieces of magnet 24 and causes a reversal of polarity in the bar magnet, thus " marking " the tray. As the reaction when this occurs tends to withdraw the catches and release the tray bottom, a plate is arranged to retain the tray bottoms in place at this point. The final magnet 25 is also a permanent magnet and releases the bottoms of all the marked trays by magnetic reaction above conveyer 7. Unmarked trays are not affected and emptied trays are closed on reaching drum 4 at the end of the conveyer where the full trays are emptied. The photo-sensitive detectors 6 are preferably of the germanium type having a narrow straight line light-sensitive junction. Each detector 6 is mounted in a brass tube 35 having an adjustable extension 36 carrying a lens 37. The tube 36 is adjusted so that rays from the material being examined are brought to a focus at the junction. In order to increase the width viewed by the detector a semi-cylindrical lens 38 is provided. The material in the trays is illuminated diffusely to avoid specular reflection effects and all the detectors 6 are arranged in parallel in a circuit arranged to avoid spurious effects due to temperature variations (Figs. 8 and 9, not shown). The output from the viewing detectors 6 is applied to an amplifier 65 and then squared at 66 before application to side 59a of a bi-stable circuit 59. To sense the passing of the trays a further photo-cell 53 is arranged to one side of the conveyer and above the level of the trays. It is illuminated by a lamp 54 on the opposite side of the conveyer except when small screens 54a mounted on the ends of the trays intervene. These screens extend longitudinally from the inside back of one tray to the inside front of the next so that the output of cell 53 can also be used to disable the bank of cells 6 between trays. Cell 53 feeds amplifier 55 whose output signals are squared at 56 and fed through gate circuit 72 for this purpose. The signals from unit 56 are also fed to differentiating circuit 57 followed by rectifying circuit 58 to provide sharp impulses occurring as the end of each tray reaches the scanning position. These impulses are applied to the second side 59b of circuit 59 to return it to its initial state if it has been changed over by a signal from the cells 6 indicating the presence of unwanted material in the tray. A potential derived from side 59a of circuit 59 is fed to side 62a of a second bi-stable circuit 62 after differentiation, the pulses of polarity corresponding to the return of circuit 59 to its initial state at the end of the scan of a tray being effective to cause the circuit 62 to change-over. It is returned to its initial state (if necessary) by a pulse applied to side 62b at the commencement of the scan of the next succeeding tray obtained by differentiating, inverting, and rectifying an output from unit 56 in circuits 60 and 61. During the times when circuit 62 is in its second state, which times occur for the duration of the signal from cell 53 occurring immediately after the passage of trays containing unwanted material, power amplifier 70 is energized to apply current to coil 27 to mark such trays for subsequent release by magnet 25 as previously described. By including a further bi-stable circuit it is possible to arrange that a tray is only marked if it contains two or more nodules of unwanted material (Figs. 6 and 7, not shown) and by further additions to the circuit it is possible to arrange for rejection only when the tray contains more than a predetermined number of unwanted nodules. The reset pulses applied to circuits 59 and 60 may be derived from a photo-sensitive source different from the cell 53 described and may be arranged to give a longer energization period for coil 27 so as to make the placing of magnet 24 less critical and, if desired, alternate trays may be arranged to form separate series controlled by different means. The photo-sensitive devices 6 may be replaced by detectors responsive to the radio-active properties of the material.